body, html {
  margin: 0;
  padding: 0;
  background-image: url(bj.png);
  background-repeat: no-repeat;
  background-size: cover;
  width: 100%;
  height: 100%;
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
}

#el {
  width: 6rem;
  height: 6rem;
  background-color: blue;
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  margin: auto;
  -webkit-animation: change-color 6s infinite ease-in-out;
          animation: change-color 6s infinite ease-in-out;
  border-radius: .4rem;
  border: .1rem solid #3fa;
}

#log {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  pointer-events: none;
  z-index: 100;
}

@-webkit-keyframes change-color {
  0% {
    background-color: blue;
  }
  50% {
    background-color: #f09;
  }
  100% {
    background-color: blue;
  }
}

@keyframes change-color {
  0% {
    background-color: blue;
  }
  50% {
    background-color: #f09;
  }
  100% {
    background-color: blue;
  }
}
